The invention relates to a process for the benzylation of p-substituted phenols in the o-position.
It is known that mixtures of 2-benzyl-, 4-benzyl- and 2,6-dibenzyl-phenols and higher-boiling condensation products are always obtained in varying composition during the benzylation of phenols.
It is thus known, for example, from J. Amer. Chem. Soc. 53, 2379 (1931) that a product mixture which, depending on the ratio of the amounts of the starting substances, contains 30 to 36% by weight of dibenzylated secondary and side reaction products is formed in the condensation of p-cresol with benzyl alcohol in the presence of aluminium chloride.
It is furthermore known [Abdurasuleva et al., Zh. Org. Khim. 9, 132 (1973)] that up to 18 mol % of dibenzyl chlorophenols are formed in the benzylation of 4-chlorophenols with benzyl chloride in the presence of catalysts such as FeCl.sub.3, FeSO.sub.4 or ZnSO.sub.4. Moreover, the condensation of 4-chlorophenol with benzyl chloride in a molar ratio of 4:1 in the presence of 10% by weight of a sulphonated styrene/divinylbenzene copolymer (strongly acid cation exchanger) to give 2-benzyl-4-chlorophenol is described in Czech Patent Specification 170,972 [Chemical Abstracts 89, (1978), 6109 b]. The yield of the desired product is indeed 83%, based on the 4-chlorophenol reacted; however, a relatively large amount of higher-boiling secondary and side reaction products is also formed; the ratio of the 2-benzyl-4-chlorophenol formed to the higher-boiling products is 4.16:1.
As well as having a low selectivity and therefore a low utilisation of the starting materials, the processes described require a large excess of phenol and long reaction times because of a slow introduction of the benzylating agent and an after-reaction period (for example 3+2=5 hours in CS 170,972), which means that a low space/time yield is achieved Furthermore, a large amount of catalyst has to be separated off and regenerated or destroyed.
Zeolites of the faujasite type are described in EP 278,211 as catalysts for the reaction of p-substituted phenols with optionally substituted benzylating agents. Using 10% by weight of Na-Y as the catalyst, 1 mol of benzyl alcohol can be reacted quantitatively with 4 mol of 4-chlorophenol within 3 hours at 200.degree. C., the ratio of 2-benzyl-4-chlorophenol formed to higher-boiling products of 9.9:1 being significantlly better than that in CS 170,972. However, the removal and, where appropriate, regeneration of the catalyst employed is technically cumbersome.